Electrode materials with high surface conductivity

ABSTRACT

The present invention concerns electrode materials capable of redox reactions by electrons and alkaline ions exchange with an electrolyte. The applications are in the field of primary (batteries) or secondary electrochemical generators, super capacitors and light modulating system of the super capacitor type.

FIELD OF INVENTION

The present invention concerns electrode materials capable of redoxreactions by electrons and alkaline ions exchange with an electrolyte.The applications are in the field of primary (batteries) or secondaryelectrochemical generators, supercapacitors and light modulating systemof the electrochromic type.

BACKGROUND OF THE INVENTION

Insertion compounds (hereinafter also referred to as electroactivematerials or redox materials) are well known, and their operation isbased on the exchange of alkaline ions, in particular lithium ions, andvalence electrons of at least one transition element, in order to keepthe neutrality of the solid matrix. The partial or complete maintenanceof the structural integrity of the material allows the reversibility ofthe reaction. Redox reactions resulting in the formation of severalphases are usually not reversible, or only partially. It is alsopossible to perform the reactions in the solid phase through thereversible scission of the sulphur-sulphur bonds or the redox reactionsinvolved in the transformation of the aromatic organic structures inquinonoid form, including in conjugated polymers.

The insertion materials are the electrochemical reactions activecomponents used in particular in electrochemical generators,supercapacitors or light transmission modulating system (electrochromicdevices).

The progression of the ions-electrons exchange reaction requires theexistence within the insertion material of a double conductivity,simultaneous with the electrons and the ions, in particular lithiumions, either one of these conductivities being eventually too weak toensure the necessary kinetic exchanges for the use of the material, inparticular for electrochemical generators or supercapacitors. Thisproblem is partly solved by using so-called “composite” electrodes,wherein the electrode material is dispersed in a matrix containing theelectrolyte and a polymer binder. When the electrolyte is a polymerelectrolyte or a polymer gel working in the presence of a solvent, themechanical binding role is carried out directly by the macromolecule.Gel means a polymer matrix, solvating or not, and retaining a polarliquid and a salt, to confer to the mixture the mechanical properties ofa solid while retaining at least a part of the conductivity of the polarliquid. A liquid electrolyte and the electrode material can also bemaintained in contact with a small fraction of an inert polymer binder,i.e., not interacting with the solvent. With any of these means, eachelectrode material particle is thus surrounded by an electrolyte capableof bringing the ions in direct contact with almost the totality of theelectrode material surface. To facilitate electronic exchanges, it isusual, according to the prior art, to add particles of a conductivematerial to one of the mixtures of the electrode material andelectrolyte mentioned above. Such particles are in a very divided state.Generally, carbon-based materials are selected, and especially carbonblacks (Shawinigan or Ketjenblack®). However, the volume fractions usedmust be kept low because such material modifies strongly the rheologytheir suspension, especially in polymers, thereby leading to anexcessive porosity and loss of operating efficiency of the compositeelectrode, in terms of the fraction of the usable capacity as well asthe kinetics, i.e, the power available. At these low concentrationsused, the carbon particles structure themselves in chains, and thecontact points with the electrode materials are extremely reduced.Consequently, such configuration results in a poor distribution of theelectrical potential within the electroactive material. In particular,over-concentrations or depletion can appear at the triple junctionpoints:

These excessive variations of the mobile ions local concentrations andthe gradients within the electroactive materials are extremelyprejudicial to the reversibility of the electrode operation over a highnumber of cycles. These chemical and mechanical constraints or stresses,result at the microscopic level in the disintegration (particulation) ofthe electroactive material particles, a part of which being susceptibleto lose the contact with the carbon particles and thus becomingelectrochemically inactive. The material structure can also bedestroyed, with the appearance of new phases and eventual release oftransition metal derivatives, or other fragments in the electrolyte.These harmful phenomenons appear even more easily the larger the currentdensity or the power requested at the electrode is.

IN THE DRAWINGS

FIG. 1 illustrates the difference between a classic electrode accordingto the prior art (A) and an electrode according to the invention whereinthe electroactive material particles are coated with a carbonaceouscoating (B).

FIGS. 2 and 3 illustrate a comparison between a sample of LiFePO₄ coatedwith a carbonaceous deposit, with an uncoated sample. The results wereobtained by cyclic voltammetry of LiFePO₄/POE₂₀LiTFSI/Li batteriescycled at 20 mV·h⁻¹ between 3 and 3.7 volts at 80° C. The first cycle isshown on FIG. 2, and the fifth on FIG. 3.

FIG. 4 illustrates the evolution of capacity during cycling forbatteries containing carbonaceous and non-carbonaceous LiFePO₄ samples.

FIG. 5 illustrates the performances of a battery containing carbonaceousLiFePO₄ and cycled under an intentiostatic mode between 3 and 3.8 V at80° C. with a charge and discharge speed corresponding to C/1.

FIG. 6 illustrates the evolution of the current vs. time of aLiFePO₄/gamma-butyrolactone LiTFSI/Li containing a carbonaceous sampleand cycled at 20 mV·h⁻¹ between 3 and 3.7 V at room temperature.

FIG. 7 illustrates the evolution of the current vs. time of aLiFePO₄/POE₂₀LiTFSI/Li containing a carbonaceous sample.

FIGS. 8 and 9 illustrate a comparison between carbonaceous andnon-carbonaceous LiFePO₄ samples cycled. The results have been obtainedby cyclic voltammetry of LiFePO₄/POE₂₀LiTFSI/Li batteries cycled at 20mV·h⁻¹ between 3 and 3.7 volts at 80° C. The first cycle is shown onFIG. 8, and the fifth on FIG. 9.

FIG. 10 illustrates the evolution of the capacity during cycling ofbatteries prepared with carbonaceous and non-carbonaceous LiFePO₄samples.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an electrodematerial comprising a complex oxide corresponding to the general formulaA_(a)M_(m)Z_(z)O_(o)N_(n)F_(f) wherein:

-   -   A comprises an alkaline metal;    -   M comprises at least one transition metal, and optionally at        least one non-transition metal such as magnesium or aluminum;        and mixtures thereof;    -   Z comprises at least one non-metal;    -   O is oxygen; N nitrogen and F is fluorine; and    -   the coefficients a, m, z, o, n, f≧0 and are selected to ensure        electroneutrality, wherein a conductive carbonaceous material is        deposited homogeneously on a surface of the material to obtain a        substantially regular electric field distribution on the surface        of material particles. The similarity in ionic radii between        oxygen, fluorine and nitrogen allows mutual replacement of these        elements as long as electroneutrality is maintained. For        simplicity and considering that oxygen is the most frequently        used element, these materials are thereafter referred to as        complex oxides. Preferred transition metals comprise iron,        manganese, vanadium, titanium, molybdenum, niobium, tungsten,        zinc and mixtures thereof. Preferred non-transition metals        comprises magnesium and is aluminum, and preferred non-metals        comprise sulfur, selenium, phosphorous, arsenic, silicon,        germanium, boron, and mixtures thereof.

In a preferred embodiment, the final mass concentration of thecarbonaceous material varies between 0.1 and 55%, and more preferablybetween 0.2 and 15%.

In a further preferred embodiment, the complex oxide comprises sulfates,phosphates, silicates, oxysulfates, oxyphosphates, oxysilicates andmixtures thereof, of a transition metal and lithium, and mixturesthereof. It may also be of interest, for structural stability purposes,to replace partially the transition metal with an element having thesame ionic radius but not involved in the redox process. For example,magnesium and aluminum, in concentrations preferably varying between 1and 25%.

The present invention also concerns electrochemical cells wherein atleast one electrode is made of an electrode material according to thepresent invention. The cell can operate as a primary or secondarybattery, a supercapacitor, or a light modulating system, the primary ora secondary battery being the preferred mode of operation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention allows the fabrication of electrode materials ofextremely varied compositions with its surface, or most of it, coatedwith a uniform coating of a conductive carbonaceous material depositedchemically. The presence in the electrode materials of the invention ofa uniform coating, when compared to a punctual contact obtained withcarbon powders or other prior art conductive additives, allows a regulardistribution of the electrical field at the surface of the electroactivematerial particles. Further, the ions concentration gradients areconsiderably diminished. Such improved distribution of theelectrochemical reaction at the surface of the particles allows on oneside the maintenance of the structural integrity of the material, and onthe other side improves the kinetics in terms of the current density andpower availability at the electrode, because of the greater surfaceaccessible.

In the present application, carbonaceous material means a solid polymercomprising mainly carbon, i.e. from 60 to 100% molar, and having anelectronic conductivity higher than 10⁻⁶ S/cm at room temperature,preferably higher than 10⁻⁴ S/cm. Other elements that can be present arehydrogen, oxygen, nitrogen, as long as they do not interfere with thechemical inertia of the carbon during the electrochemical operation. Thecarbonaceous material can be obtained through thermal decomposition ordehydrogenation, e.g., by partial oxidation, of various organicmaterials. In general, any material leading, through a reaction or asequence of reactions to the solid carbonaceous material with thedesired property without affecting the stability of the complex oxide,is a suitable precursor. Preferred precursors include, but are notlimited to: hydrocarbons and their derivatives, especially thosecomprising polycyclic aromatic moieties, like pitch and tar derivatives,perylene and its derivatives; polyhydric compounds like sugars andcarbon hydrates and their derivatives; and polymers. Preferred examplesof such polymers include polyolefins, polybutadienes, polyvinylicalcohol, phenol condensation products, including those from a reactionwith an aldehyde, polymers derived from furfurylic alcohol, polymersderivatives of styrene, divinylbenzene, naphtalene, perylene,acrylonitrile, vinyl acetate; cellulose, starch and their esters andethers, and mixtures thereof.

The improvement of the conductivity at the surface of the particlesobtained with the carbonaceous material coating according to the presentinvention allows the satisfactory operations of electrodes containingelectroactive materials having an insufficient electronic conductivityto obtain acceptable performances. Complex oxides with redox couples inuseful voltage range and/or using elements inexpensive or nontoxic butwhose conductivity otherwise would be to low for practical use, nowbecome useful as electrode materials when the conductive coating ispresent. The choice of the structures or phase mixtures possessing redoxproperties but having an electronic conductivity that is too low, isthus much wider than that with compounds of the prior art, of thelithium and transition metal mixed oxides. It is particularly possibleto include within the redox structures, at least one element selectedfrom non-metals (metalloids) such as sulphur, selenium, phosphorus,arsenic, silicon or germanium, wherein the greater electronegativityallows the modulation of the redox potential of the transition elements,but at the expense of the electronic conductivity. A similar effect isobtained with the partial or complete substitution of the oxygen atomswith fluorine or nitrogen.

The redox materials are described by the general formulaA_(a)M_(m)Z_(z)O_(o)N_(n)F_(f) wherein:

-   -   A comprises an alkaline metal such as Li, Na, or K;    -   M comprises at least one transition metal, and optionally at        least one a non-transition metal such as magnesium or aluminum;        or mixtures thereof;    -   Z comprises at least one non-metal such as S, Se, P, As, Si, Ge,        B;    -   O is oxygen;    -   N is nitrogen and F is fluorine, wherein the latter elements can        replace oxygen in the complex oxide because of the ionic radii        values for F^(−, O) ²⁻ and N³⁻ are similar; and    -   each coefficient a, m, z, o, n and f≧0 independently, to ensure        electroneutrality of the material.

Preferred complex oxides according to the invention comprise those offormula Li_(1+x)MP_(1−X)Si_(x)O₄; Li_(1+x−y)MP_(1-x)Si_(x)O_(4−y)F_(y);Li_(3−x+z)M₂(P_(1−x−z)S_(x)Si_(z)O₄)₃;Li_(3+u−x+z)V_(2−z−w)Fe_(u)Ti_(w)(P_(1−x−z)S_(x)Si_(z)O₄)₃, orLi_(4+x)Ti₅O₁₂, Li_(4+x−2y)Mg_(y)TiO₁₂, wherein w≦2; 0≦x, y≦1; z≦1 and Mcomprises Fe or Mn.

The carbonaceous coating can be deposited through various techniquesthat are an integral part of the invention. A preferred method comprisesthe pyrolysis of organic matter, preferably carbon-rich, in the presenceof the redox material. Particularly advantageous are mesomolecules andpolymers susceptible of easily forming, either mechanically or byimpregnation from a solution or through in situ polymerization, auniform layer at the surface of the redox material particles. Asubsequent pyrolysis or dehydrogenation step thereof provides a fine anduniform layer of the carbonaceous material at the surface of theparticles of the redox material. To ensure that the pyrolysis ordehydrogenation reaction will not affect the latter, it is preferred toselect compositions wherein the oxygen pressure liberated from thematerial be sufficiently low to prevent oxidation of the carbon formedby the pyrolysis. The activity of the oxygen of compoundsA_(a)M_(m)Z_(z)O_(o)N_(n)F_(f) can be controlled by the concentration ofalkaline metal, which itself determines the oxidation state of thetransition element or elements contained in the material and being apart of the invention. Of particular interest are the compositionswherein the coefficient “a” of the alkaline metal concentration allowsthe maintenance of the following oxidation states: Fe²⁺, Mn²⁺, V²⁺, V³⁺,Ti²⁺, Ti³⁺, Mo³⁺, Mo⁴⁺, Nb³⁺, Nb⁴⁺, W⁴⁺. Generally, oxygen pressures inthe order of 10⁻²⁰ bars at 0° C. and de 10⁻¹⁰ bars at 900° C. aresufficiently low to allow the deposition of carbon by pyrolysis, thekinetic of carbon formation in the presence of hydrocarbonaceousresidues resulting from the pyrolysis being quicker and less activatedthan oxygen formation from the redox materials. It is also possible andadvantageous to select materials having an oxygen pressure inequilibrium with the materials that is inferior to that of theequilibriumC+O₂

CO₂

In this instance, the carbonaceous material can be thermodynamicallystable vis-à-vis the complex oxide. The corresponding pressures areobtained according to the following equation:${\ln\quad{P\left( O_{2} \right)}} = {{\ln\quad{P\left( {CO}_{2} \right)}} = \frac{94050}{R\left( {273,{2 + \theta}} \right)}}$wherein R is the perfect gas constant (1,987 cal·mole⁻¹·K⁻¹); and θ isthe temperature in ° C.

Table 1 provides oxygen pressures at several temperatures: P(O₂) P(O₂) θ(° C.) P(CO₂) = 1 atm P(CO₂) = 10⁻⁵ atm 200 3.5 × 10⁻⁴⁴ 3.5 × 10⁻⁴⁹ 3001.4 × 10⁻³⁶ 1.4 × 10⁻⁴¹ 400 2.9 × 10⁻³¹ 2.9 × 10⁻³⁶ 500 2.5 × 10⁻²⁷ 2.5× 10⁻³² 600 2.9 × 10⁻²⁴ 2.5 × 10⁻²⁹ 700 7.5 × 10⁻²² 7.5 × 10⁻²⁷ 800 7.0× 10⁻²⁰ 7.0 × 10⁻²⁵ 900 3.0 × 10⁻¹⁸ 3.0 × 10⁻²³

It is also possible to perform the carbon deposition through thedisproportionation of carbon oxide at temperatures lower than 800° C.according to the equation:2CO

C+CO₂

This reaction is exothermic but slow. The complex oxide particles can becontacted with carbon monoxide pure or diluted in an inert gas, attemperatures varying from 100 to 750° C., preferably between 300 and650° C. Advantageously, the reaction is carried out in a fluidized bed,in order to have a large exchange surface between the gaseous phase andthe solid phase. Elements and cations of transition metals present inthe complex oxide are catalysts of the disproportionation reaction. Itcan be interesting to add small amounts of transition metal salts,preferably iron, nickel, or cobalt, at the surface of the particles,these elements being particularly active as catalysts of thedisproportionation reaction. In addition to carbon monoxidedisproportionation, hydrocarbons in the gaseous forms can be decomposedat moderate to high temperature to yield carbon deposits. Of specialinterest for the operation are the hydrocarbons with a low energy offormation, like alkenes, alkynes or aromatic rings.

In a variation, the deposition of the carbonaceous material can beperformed simultaneously with a variation of the composition of alkalinemetals A. To do so, an organic acid or polyacid salt is mixed with thecomplex oxide. Another possibility comprises the in situ polymerizationof a monomer or monomer mixtures. Through pyrolysis, the compounddeposits a carbonaceous material film at the surface and the alkalinemetal A is incorporated according to the equation:A_(a′)M_(m)Z_(z)O_(o)N_(n)F_(f)+A_(a−a′)C_(c)O_(o)R′

A_(a)M_(m)Z_(z)O_(o)N_(n)F_(f)

-   -   R′ being an organic radical, eventually part of a polymeric        chain.

Among the compounds susceptible of permitting this reaction, it can bementioned in a non limited manner salts of carboxylic acids such asoxalic, malonic, succinic, citric, polyacrylic, polymethacrylic,benzoic, phtalic, propiolic, acetylene dicarboxylic, naphthalene di- ortetracarboxylic, perylene tetracarboxylic and diphenic acids.

Obviously, the pyrolysis of an organic material deprived of an alkalineelement in combination with an alkaline element salt can also lead tothe desired stoichiometry of the complex oxide.

It is also possible to obtain a carbonaceous material deposit,especially at low or mid-range temperatures, lower than 400° C., byreduction of carbon-halogen bonds according to the equation:CY—CY+2e ⁻

-C═C-+2Y⁻wherein Y represents a halogen or a pseudo-halogen. The termpseudo-halogen means an organic or inorganic radical susceptible ofexisting in the form of an ion Y⁻ and formed a corresponding protonatedcompound HY. Examples of halogen and pseudo-halogen include F, Cl, Br,I, CN, SCN, CNO, OH, N₃, RCO₂, RSO₃ wherein R is H or an organicradical. The formation by reduction of CY bonds is preferably performedin the presence of reducing elements such as hydrogen, zinc, magnesium,Ti³⁺ ions, Ti²⁺ ions, Sm²⁺ ions, Cr²⁺ ions, V²⁺ ions,tetrakis(dialkylamino ethylene) or phosphines. These reagents caneventually be obtained or regenerated electrochemically. Further, it canalso be advantageous to use catalysts increasing the reduction kinetic.Palladium or nickel derivatives are particularly efficient, particularlyin the form of complexes with phosphorous or nitrogen compounds like2,2′-bipyridine. Similarly, these compounds can be generated chemicallyin an active form in the presence of reducing agents, such as thosementioned above, or electrochemically. Compounds susceptible ofgenerating carbon by reduction include perhalocarbons, particularly inthe form of polymers, hexachlorobutadiene and hexachlorocyclopentadiene.

Another way to free carbon from a low temperature process comprises theelimination of the hydrogenated compound HY, Y being as defined above,according to the equation:—CH—CY-+B

-C═C-+BHY

Compounds susceptible of generating carbon from reduction includeorganic compounds comprising an even number of hydrogen atoms and Ygroups, such as hydrohalocarbons, in particular in the form of polymers,such as vinylidene polyfluoride, polychloride or polybromide, or carbonhydrates. The dehydro (pseudo) halogenation can be obtained at lowtemperature, including room temperature, by reacting a base with the HYcompound to form a salt. Example of suitable bases include tertiarybasis, amines, amidines, guanidines, imidazoles, inorganic bases such asalkaline hydroxides, organometallic compounds behaving like strongbases, such as A(N(Si(CH₃)₃)₂, LiN[CH(CH₃)₂]₂, and butyl-lithium.

In the last two methods, it can be advantageous to anneal the materialafter the carbon deposition. Such treatment improves the structure orthe crystallinity of the carbonaceous deposit. The treatment can beperformed at a temperature varying between 100 and 1000° C., preferablybetween 100 and 700° C., to prevent the potential reduction of thecomplex oxide by the carbonaceous material.

Generally, it is possible to obtain uniform carbonaceous materialcoatings, ensuring a sufficient electronic conductivity, i.e. at leaston the same order than the ionic conductivity of the oxide particle. Thethick coatings provide a conductivity sufficient so that the binarymixture of complex oxide particles coated with the carbonaceousmaterial, and the electrolyte, liquid or polymeric or the inertmacromolecular binder to be wetted with the electrolyte, is conductiveby a simple contact between the particles. Generally, such behaviour canbe observed at volumic fractions comprised between 10 and 70%.

It can also be advantageous to select deposits of carbonaceous materialssufficiently thin to prevent obstruction of the passage of ions, whileensuring the distribution of the electrochemical potential at thesurface of the particles. In this instance, the binary mixtureseventually do not possess an electronic conductivity sufficient toensure the electronic exchanges with the electrode substrate (currentcollector). The addition of a third electronic conductive component, inthe form of a fine powder or fibers, provides satisfactory macroscopicconductivity and improves the electronic exchanges with the electrodesubstrate. Carbon blacks or carbon fibers are particularly advantageousfor this purpose and give satisfactory results at volumic concentrationsthat have little or no effect on the rheology during the use of theelectrode because of the existence of electronic conductivity at thesurface of the electrode material particles. Volumic fractions of 0.5 to10% are particularly preferred. Carbon black such as Shawinigan® orKetjenblack® are preferred. Among carbon fibers, those obtained bypyrolysis of polymers such as tar, pitch, polyacrylonitrile as well asthose obtained by cracking of hydrocarbons are preferred.

Interestingly, because of its light weight and malleability, aluminiumis used as the current collector constituent. This metal is nonethelesscoated with an insulating oxide layer. This layer, which protects themetal from corrosion, can in certain conditions increase the thickness,leading to an increase resistance of the interface, prejudicial to thegood operation of the electrochemical cell. This phenomenon can beparticularly detrimental and fast when the electronic conductivity isonly ensured, as in the prior art, by the carbon particles having alimited number of contact points. The use, in combination withaluminium, of electrode materials coated with a conductive carbonaceousmaterial layer increases the exchange surface aluminium-electrode. Thealuminium corrosion effects are therefore cancelled or at leastsignificantly minimized. It is possible to use either aluminiumcollectors in the form of sheet or eventually in the form of expanded orperforated metal or fibers, which allow a weight gain. Because of theproperties of the materials of the invention, even in the case ofexpended or perforated metal, electronic exchanges at the collectorlevel take place without noticeable increase of the resistance.

Whenever the current collectors are thermally stable, it is alsopossible to perform the pyrolysis or dehydrogenation directly on thecollector, to obtain after carbon deposition, a continuous porous filmthat can be infiltrated with an ionic conductive liquid, or with amonomer or a mixture of monomers generating a polymer electrolyte afterin situ polymerisation. The formation of porous films in which thecarbonaceous coating forms a chain are easily obtained according to theinvention through pyrolysis of a complex oxide-polymer compositedeposited in the form of a film on a metallic substrate.

In using the electrode material according to the invention into anelectrochemical cell, preferably a primary or secondary battery, theelectrolyte is preferably a polymer, solvating or not, optionallyplasticized or gelled by a polar liquid comprising in solution one ormore metallic salts, preferably at least a lithium salt. In suchinstance, the polymer is preferably formed from units of oxyethylene,oxypropylene, acrylonitrile, vinylidene fluoride, acrylic acid ormethacrylic acid esters, itaconic acid esters with alkyls or oxaalkylgroups. The electrolyte can also be a polar liquid immobilized in amicroporous separator, such as a polyolefin, a polyester, nanoparticlesof silica, alumina or lithium aluminate LiAlO₂. Examples of polarliquids include cyclic or linear carbonates, alkyl formiates,oligoethylene glycols, α-ω alkylethers, N-methylpyrrolidinone,γ-butyrolactone, tetraalakylsulfamides and mixtures thereof.

The following examples are provided to illustrate preferred embodimentof the invention, and shall not be construed as limiting its scope.

EXAMPLE 1

This example illustrates the synthesis of a material of the presentinvention leading directly to an insertion material coated with acarbonaceous deposit.

The material LiFePO₄ coated with a carbonaceous deposit is prepared fromvivianite Fe₃(PO₄)₂.8H₂O and lithium orthophosphate Li₃PO₄ instoichiometric amounts according to the reaction:Fe₃(PO₄)₂.8H₂O+Li₃PO₄=>3 LiFePO₄

Polypropylene powder in an amount corresponding to 3% by weight ofvivianite is added. The three components mixed together intimately andground in a zirconia ball mill. The mixture is then heated under aninert atmosphere of argon, firstly at 350° C. for 3 hours to dehydratethe vivianite. Subsequently, the temperature is risen gradually up to700° C. to crystallize the material and carbonize the polypropylene. Thetemperature is maintained at 700° C. for 7 hours. The structure of thematerial obtained, as verified by X-rays, corresponds to that publishedfor triphyllite. The amount of carbon present in the sample has beendetermined by elemental analysis, and gave a concentration of 0.56%. Forcomparison purposes, a similar sample has been prepared in similarconditions, but without the addition of polypropylene powder. Thislatter sample also shows a pure crystalline structure of the typeLiFePO₄.

Electrochemical Properties

The materials prepared were tested in button batteries of the CR2032type at room temperature and 80° C.

Tests at 80° C. (Polymer Electrolyte)

The materials obtained above have been tested in button batteries of theCR2032 type. The cathode were obtained by mixing together the activematerial powder with carbon black (Ketjenblack®) to ensure theelectronic exchange with the current collector, and polyethylene oxidewith a molecular weight of 400 000 is added as both a binder and anionic conductor. The proportions, by weight, are 35:9:56. Acetonitrileis added to the mixture to dissolve the ethylene polyoxide. The mixtureis homogenized and poured on a stainless steel disc of 1.7 cm². Thecathode is dried under vacuum, and transferred in a Vacuum Atmospheresglove box, under helium atmosphere (<1 vpm H₂O, O₂). A sheet of lithium(27 μm) laminated on a nickel substrate is used as the anode. Thepolymer electrolyte comprises polyethylene oxide of weight 5 000 000 andLiTFSI (lithium bis-trifluoromethanesulfonimide) in proportions ofoxygen of oxyethylene units/lithium of 20:1.

The electrochemical experiments were carried out at 80° C., temperatureat which the ionic conductivity of the electrolyte is sufficient (2×10⁻³Scm⁻¹). The electrochemical studies are performed by slow voltammetry(20 mV·h⁻¹) controlled by a battery cycler of the Macpile® type. Thebatteries were charged and discharged between 3.7 and 3 V.

FIG. 2 illustrates the first cycle obtained for carbonaceous andnon-carbonaceous materials prepared above. For the non-carbonaceoussample, the oxidation and reduction phenomenons extend trail over a widepotential range. For the carbonaceous sample, the peaks are much betterdefined on a narrow potential domain. The evolution of both materialsduring the first 5 cycles is very different (FIG. 3). For thecarbon-coated sample, the oxidation and reduction kinetics become fasterand faster, thus leading to better defined peaks (larger peak currentsand narrower peak widths). However, for the non-carbonaceous sample, thekinetics become slower and slower. The evolution of the capacity of bothsamples is illustrated in FIG. 4. For the carbonaceous sample, thecapacity exchanged is stable. It represents from 94 to 100% of thetheoretical capacity (170 mAh·g⁻¹) depending on the sample. The initialcapacity of the non-carbonaceous material is around 145 mAh·g⁻¹, i.e.,about 85% of the theoretical capacity. For this sample, the capacityexchanged quickly decreases. After 5 cycles, the battery has lost 20% ofits initial capacity.

The carbonaceous sample is cycled under an intentiostatic mode between3.8 and 3 V with fast charging and discharging rates. The imposedcurrents correspond to a C/1 rate, which means that all the capacity isexchanged in 1 hour. These cycling results are shown in FIG. 5. Thefirst 5 cycles are performed under a voltamperometric mode to activatethe cathode and determine its capacity. In this instance, 100% of thetheoretical capacity is exchanged during the first voltammetric cyclesand 96% during the first 80 intentiostatic cycles. Subsequently, thecapacity slowly decreases, and after 1000 cycles, 70% of the capacity(120 mAh·g⁻¹) is still exchanged at this rate. The cycling in thepotentiodynamic mode performed after 950 cycles show that in reality,89% of the initial capacity is still available at slower dischargesrates. The loss of power is associated with the increase of theresistance at the lithium/polymer electrolyte interface. The parameter(capacity passed during charging)/(capacity passed during discharging)become erratic in appearance. This parameter C/D, shown on FIG. 5 at theend of cycling, leads to the presumption that dendrites are formed.

Tests at Room Temperature (Liquid Electrolyte)

The LiFePO₄ coated with a carbonaceous deposit was also tested at roomtemperature. In this instance, the composite cathode is prepared bymixing the active material with carbon black and EPDM (preferablydissolved in cyclohexane) in ratio 85:5:10. The mixture is spread onto astainless steel current collector in the form of a disc of 1.7 cm²,dried under vacuum, and kept in a glove box under helium atmosphere. Asabove, lithium is used as the anode. Both electrodes are separated by aCelgard™ porous membrane. The electrolyte used in a LiTFSI 0.8 molalsolution in gamma-butyrolactone. The voltamperograms illustrated in FIG.6 were obtained at room temperature under is slow voltammetry (20mV·h⁻¹) between 3 and 3.8 V. With such configuration, the oxidation andreduction kinetics appears to be much slower than at 80° C. Further, thepower of the battery decreases slowly during cycling. On the other hand,the entire theoretical capacity is accessible (97.5% cycle 1, 99.4%cycle 5), i.e., reversibly exchanged without loss during cycling (5cycles). It is not excluded that the low power of this battery may comefrom a poor permeation of the electrode by the electrolyte, the latterbeing a poor wetting agent for the binding polymer.

The example illustrates that the improvement of the material studied,because of the presence of the carbonaceous deposit at the surface ofthe particles, is reflected on the kinetics, the capacity and thecyclability. Further, its role is independent from that of the type ofcarbon black added during the preparation of composite cathodes.

EXAMPLE 2

This example shows the formation of a conductive carbonaceous depositfrom a hydrocarbon gas. The synthesis described in Example 1 for thepreparation of lithium iron phosphate is repeated without addingpolypropylene powder, and by replacing the thermal treatment inertatmosphere with a mixture of 1% propene in nitrogen. During the thermaltreatment, propene decomposes to form a carbon deposit on the materialbeing synthesized. The resulting sample obtained contains 2.5% ofcarbon, as determined by chemical analysis. Cyclic voltammetry isperformed on this sample under the conditions described in Example 1 decarbon, and shows the important activation phenomenon during the firstcycles (see FIG. 6). The improvement in redox kinetics is accompanied inthis instance by an increase of the capacity reversibly exchanged. Asmeasured during the discharge step, the initial capacity of the LiFePO₄sample prepared represents 77% of the theoretical capacity, taking intoaccount the 2.5% electrochemically inactive carbon. After 5 cycles, thecapacity reaches 91.4%. The activation phenomenon observed is linked tothe thickness of the carbon layer eventually porous coating theparticles and capable of slowing the diffusion of the cations.

The following examples 3-5 illustrate the treatment of the complexoxide, namely the lithium iron phosphate LiFePO₄, prepared thermally andindependently in order to obtain a conductive carbonaceous coating.

EXAMPLE 3

The tryphilite sample LiFePO₄ prepared above is analyzed. Its masscomposition is: Fe: 34.6, Li: 4.2%, P: 19.2, which represents a 5%difference with respect to the stoichiometry.

The powder to be treated is impregnated with an aqueous solution ofcommercial sucrose, and dried. The amount of solution is selected tocorrespond to 10% of the weight of sucrose with respect to the weight ofthe material to be treated. Water is completely evaporated underagitation to obtain a homogeneous distribution. The use of sugarrepresents a preferred embodiment because it melts before beingcarbonized, thereby provided a good coating of the particles. Itsrelatively low carbon yield after pyrolysis is compensated by its lowcost.

The thermal treatments are performed at 700° C. under argon atmosphere.The temperature is maintained for 3 hours. Elemental analysis shows thatthis product contains 1.3% by weight of carbon. Such thermal treatmentleads to a black powder giving an electronic conductivity measurablewith a simple commercial ohm-meter. Its electroactivity, as measured onthe 1^(st) (FIG. 8) and 5^(th) (FIG. 9) charge-discharge cycle, is 155.9mAhg⁻¹ and 149.8 mAhg⁻¹ respectively, which is 91.7% and 88.1% of thetheoretical value. These values are to be compared with that of theproduct not coated with the carbon deposit, that has only 64%electroactivity. After 5 cycles, this value fades to 37.9% (FIG. 10).

EXAMPLE 4

Cellulose acetate is added to the phosphate LiFePO₄ of Example 3 as aprecursor of the carbon coating. This polymer is known to decompose withhigh carbonization yields, on the order of 24%. It decomposes between200 and 400° C. Above this temperature, the amorphous carbon rearrangesto give a graphite-type structure that favors coherent and highlyconductive carbon deposits.

Cellulose acetate is dissolved in acetone in a ratio corresponding to 5%by weight of the material to be treated, and dried before proceeding asabove. The carbon concentration of the final product is 1.5%. Thethermal treatment leads, in a similar manner, to a black powder havingsurface electronic conductivity. Its electroactivity, as measured on the1^(st) (FIG. 8) and 5^(th) (FIG. 9) charge-discharge cycles, is 152.6mAhg⁻¹ and 150.2 mAhg⁻¹ respectively, which is 89.8% and 88.3% of thetheoretical value. This value is to be compared with that of the productnot coated with the carbon deposit, that has only 64% electroactivity.After 5 cycles, this value fades to 37.9% (FIG. 10).

EXAMPLE 5

Perylene and its derivatives are known to lead, after pyrolysis, tographitic-type carbons, because of the existence of condensed cycles inthe starting molecule. In particular, the perylene-tetracarboxylic acidanhydride decomposes above 560° C. and provides very coating carboncoatings. However, this product shows a poor solubility, and theirintimate mixture with the complex oxide, here also LiFePO₄ of Example 3,is difficult to embody. To solve this problem, a polymer containingperylene groups separated with an ethylene polyoxide chain has beenprepared in a first step. The oxyethylene segments are selected to besufficiently long to act as solubilizing agents for the aromatic groupsin the usual organic solvents. Therefore, commercial3,4,9,10-perylenetetracarboxylic acid anhydride (Aldrich) is reactedwith Jeffamine 600 (Hunstmann) at high temperature, according to thefollowing reaction:

whereinR═—[CH(CH₃)CH₂O-]p(CH₂CH₂O)_(q)[CH₂—CH(CH)₃O]_(p-1)CH₂—CH(CH)₃-1≦p≦2;10≦n≦14The synthesis is completed within 48 hours in dimethylacetamide underreflux (166° C.). The polymer formed is precipitated in water, and asolid-liquid separation is carried out. It is purified by dissolution inacetone, followed by re-precipitation in ether. The process allows theremoval of unreacted starting materials, as well as low mass products.The powder is finally dried.

Carbonization yield of this product is of the order of 20%. The polymeris dissolved in dichloromethane in a ratio corresponding to 5% of theweight of the material to be treated before proceeding as describedabove in Examples 3 and 4. The carbon content of the final product is1%. The thermal treatment leads, as described above, to a blackconductive powder. Its electroactivity, as measured on the 1^(st) (FIG.8) and 5^(th) (FIG. 9) charge-discharge cycles, is 148.6 mAhg⁻¹ and146.9 mAhg⁻¹ respectively, which is 87.4% and 86.4% of the theoreticalvalue. This value is to be compared with that of the product not coatedwith the carbon deposit, that has only 64% electroactivity. After 5cycles, this value fades to 37.9% (FIG. 10).

EXAMPLE 6

This example illustrates the use of an elimination reaction from apolymer to form a carbonaceous deposit according of the invention.

Ferric iron sulfate Fe_(2 (SO) ₄)₃ with a “Nasicon” orthorhombicstructure was obtained from commercial hydrated iron (III) sulfate(Aldrich) by dehydration at 450° C. under vacuum. With cooling, andunder stirring, the powder suspended in hexane was lithiated withstoichiometric 2M butyl lithium to reach the compositionLi_(1.5)Fe₂(SO₄)₃. 20 g of the resulting off-white powder were slurriedin 100 ml acetone and 2.2 gram of poly(vinylidene bromide)(—CH₂CBr₂)_(n)— were added and the mixture was treated for 12 hours in aball mill with alumina balls. The suspension thus obtained was dried ina rotary evaporator and crushed as coarse powder in a mortar. The solidwas treated with 3 g of diazabicyclo [5.4.0]unde-7-cene (DBU) inacetonitrile under reflux for three hours. The black powder thusobtained was filtered to eliminate the resulting amine bromide andexcess reagent, rinsed with acetonitrile and dried under vacuum at 60°C. Further annealing of the carbonaceous deposit was performed underoxygen-free argon (<1 ppm) at 400° for three hours.

The material coated with the carbonaceous material was tested forelectrochemical activity in a lithium cell with a lithium metalelectrode, 1 molar lithium bis-(trifluoromethanesulfonimide) in 50:50ethylene carbonate-dimethoxyethane mixture as electrolyte immobilized ina 25 μm microporous polypropylene separator. The cathode was obtainedfrom the prepared redox material mixed with Ketjenblack™ and slurried ina solution of ethylene-propylene-diene polymer (Aldrich), the ratio ofsolids content being 85:10:5. The cathode mix was spread on an expandedaluminium metal grid and pressed at 1 ton cm⁻² to a resulting thicknessof 230 μm. The button cell assembly was charged (the tested materialbeing the anode) at 1 mAcm⁻² between the cut-off potentials of 2.8 and3.9 volts. The material capacity is 120 mAhg⁻¹, corresponding to 89% oftheoretical value. The average potential was obtained at 3.6 V vs.Li⁺:Li°.

EXAMPLE 7

This example illustrates the use of a nitrogen-containing compound aselectrode material.

Powdered manganous oxide MnO and lithium nitride, both commercial(Aldrich) were mixed in a dry box under helium in a 1:1 molar ratio. Thereactants were put in a glassy carbon crucible and treated under oxygenfree nitrogen (<1 vpm) at 800° C. 12 g of the resulting oxynitride withan antifluorite structure Li₃MnNO were added to 0.7 g of micrometer sizepolyethylene powder and ball-milled under helium in a polyethylene jarwith dry heptane as dispersing agent and 20 mg of Brij™ 35 (ICI) assurfactant. The filtered mix was then treated under a flow ofoxygen-free nitrogen in a furnace at 750° C. to ensure decomposition ofthe polyethylene into carbon.

The carbon-coated electrode material appears as a black powder rapidlyhydrolyzed in moist air. All subsequent handling was carried out in adry box wherein a cell similar to that of Example 6 was constructed andtested for electrochemical activity of the prepared material. Theelectrolyte in this case is a mixture of commercial tetraethylsulfamide(Fluka) and dioxolane in a 40:60 volume ratio. Both solvents werepurified by distillation over sodium hydride (under 10 torrs reducedpressure for the sulfamide). To the solvent mixture, is added lithiumbis-(trifluoromethanesulfonimide) (LiTFSI) to form a 0.85 molarsolution. Similarly to the set-up of Example 6, the cell comprises alithium metal electrode, the electrolyte immobilized in a 25 μm porouspolypropylene separator and the material processed in a way similar tothat of Example 6.

The cathode is obtained from the prepared redox material mixed withKetjenblack™ and slurried in a solution of ethylene-propylene-dienepolymer, the ratio of solids content being 90:5:5. The cathode mix ispressed on an expanded copper metal grid at 1 ton cm⁻² with a resultingthickness of 125 μm. The button cell assembly is charged at 0.5 mAcm⁻²(the oxynitride being the anode) between the cut-off potentials of 0.9and 1.8 volts. The materials capacity was 370 mAhg⁻¹, i.e., 70% of thetheoretical value for two electrons per formula unit. The averagepotential is found at 1.1 V vs. Li⁺:Li°. The material is suited for useas negative electrode material in lithium-ion type batteries. Anexperimental cell of this type has been constructed with the electrodematerial on a copper metal grid similar to that tested previously and apositive electrode material obtained by chemical delithiation of thelithium iron phosphate of Example 1 by bromine in acetonitrile. The iron(III) phosphate obtained was pressed onto an aluminium grid to form thepositive electrode and the 0.85 M LiTFSI tetraethylsulfamide/dioxolanesolution used as electrolyte. The average voltage of such cell is 2.1volts and its energy density, based on the weight of the activematerials, is 240 Wh/Kg.

EXAMPLE 8

Lithium vanadium (III) phosphosilicateLi_(3.5)V₂(PO₄)_(2.5)(SiO₄)_(0.5), having a “Nasicon” structure wasprepared in the following manner:

Lithium carbonate (13.85 g), lithium silicate Li₂SiO₃ (6.74 g),dihydrogen ammonium phosphate (43.2 g) and ammonium vanadate (35.1 g)were mixed with 250 mL of ethylmethylketone and treated in a ball millwith alumina balls in a thick-walled polyethylene jar for 3 days. Theresulting slurry was filtered, dried and treated in a tubular furnaceunder a 10% hydrogen in nitrogen gas flow at 600° C. for 12 hours. Aftercooling, 10 g of the resulting powder were introduced in a planetaryball mill with tungsten carbide balls. The resulting powder was added toa solution of the polyaromatic polymer prepared in Example 5(polyoxyethylene-co-perylenetetracarboxylicdimide 0.8 g in 5 mLacetone), well homogenized, and the solvent was evaporated.

The red-brown powder was thermolyzed in a stream of oxygen-free argon at700° C. for 2 hours, leaving after cooling a black powder with ameasurable surface conductivity. The material coated with thecarbonaceous material was tested for electrochemical activity in alithium-ion cell with a natural graphite electrode (NG7) coated on acopper current collector and corresponding to 24 mg/cm², 1 molar lithiumhexafluorophosphate in 50:50 ethylene carbonate dimethylcarbonatemixture as electrolyte immobilized in a 25 μm microporous polypropyleneseparator. The cathode was obtained from the lithium vanadiumphosphosilicate mixed with Ketjenblack™ and slurried in a solution ofvinylidenefluoride-hexafluoropropene copolymer in acetone, the ratio ofsolids content being 85:10:5. The cathode mix was spread on an expandedaluminium metal grid and pressed at 1 ton cm⁻² to a resulting thicknessof 190 μm corresponding to a active material loading of 35 mg/cm². Thebutton cell assembly was charged (the tested material being the anode)at 1 mAcm⁻² between the cut-off potentials of 0 and 4.1 volts. Thecapacity of the carbon coated material was 184 mAhg⁻¹, corresponding to78% of the theoretical value (3.5 lithium per unit formula), with a slowfading with cycling. In a comparative test, a similar cell constructedusing instead the uncoated material, as obtained after milling the heattreated inorganic precursor but omitting the addition of the perylenepolymer, shows a capacity of 105 mAhg⁻¹, rapidly fading with cycling.

EXAMPLE 9

This example illustrates the formation of a carbonaceous coatingsimultaneous to a variation of the alkali metal content of the redoxmaterial.

13.54 g of commercial iron (III) fluoride (Aldrich), 1.8 g of thelithium salt of hexa-2,4-dyine dicarboxylic acid are ball milled in athick-walled polyethylene jar with alumina balls, in the presence of 100mL of acetonitrile. After 12 hours, the resulting slurry was filteredand the dried powder is treated under a stream of dry, oxygen-freenitrogen in a tubular furnace at 700° C. for three hours. The resultingblack powder contained from elemental analysis Fe: 47%, F: 46%, Li:1.18%, C, 3.5%, corresponding to the formula Li_(0.2)FeF₃C_(0.35). Theelectrode material was tested for its capacity in a cell similar to thatof Example 6 with the difference that the cell is first tested ondischarge (the electrode material as cathode), and then recharged. Thecut-off voltages were chosen between 2.8 and 3.7.Volts. The experimentalcapacity on the first cycle was 190 mAhg⁻¹, corresponding to 83% of thetheoretical value. For comparison, a cell with FeF₃ as the electrodematerial and no carbonaceous coating has a theoretical capacity of 246mAhg⁻¹. In practice, the first discharge cycle obtained in similarconditions to the material of the invention is 137 mAhg⁻¹.

EXAMPLE 10

This example illustrates also the formation of a carbonaceous coatingsimultaneous to a variation of the alkali metal content of the redoxmaterial.

Commercial polyacrylic acid of molecular weight 15 000 was dissolved as10% solution in water/methanol mixture and titrated with lithiumhydroxide to a pH of 7. 4 μL of this solution were dried in the crucibleof a thermogravimetry air a 80° C. to evaporate the water/methanol. Theheating was then continued to 500° C., showing a residue of 0.1895 mg ofcalcination residue as lithium carbonate.

18.68 g of commercial iron (III) phosphate dihydrate, (Aldrich), 8.15 glithium oxalate (Aldrich), 39 mL of the lithium polyacrylate solution,80 mL of acetone and 40 mL of 2,2-dimethoxy acetone as water scavengerwere ball milled in a thick-walled polyethylene jar with alumina balls.After 24 hours, the resulting slurry was filtered and dried. Theresulting powder was treated under a stream of dry, oxygen-free nitrogenin a tubular furnace at 700° C. for three hours, resulting in a blackishpowder. The resulting product had the following elemental analysis: Fe:34%, P: 18.8%, Li: 4.4%, C, 3.2%. The X-ray analysis confirmed theexistence of pure triphilite LiFePO₄ as the sole crystalline component.The electrode material was tested for its capacity in a cell similar tothat of Example 1 with a PEO electrolyte, and then recharged. Thecut-off voltages were chosen between 2.8 and 3.7 V. The experimentalcapacity on the first cycle was 135 mAhg⁻¹, corresponding to 77% of thetheoretical value, increasing to 156 mAhg⁻¹ (89%) while the peakdefinition improved with further cycling. 80% of this capacity isaccessible in the potential range 3.3-3.6 V vs. Li⁺:Li°.

EXAMPLE 11

The compound LiCO_(0.75)Mn_(0.25)PO₄ was prepared from intimately groundcobalt oxalate dihydrate, manganese oxalate dihydrate and dihydrogenammonium phosphate by firing in air at 850° C. for 10 hours. Theresulting mauve powder was ball milled in a planetary mill with tungstencarbide balls to an average grain size of 4 μm. 10 g of this complexphosphate were triturated in a mortar with a 10 mL of 6% solution of theperylene polymer of Example 5 in methyl formate. The solvent rapidlyevaporated. The resulting powder was treated under a stream of dry,oxygen-free argon in a tubular furnace at 740° C. for three hours,resulting in a black powder. The electrochemical activity of the cellwas tested in a lithium-ion cell similar to that of Example 6. Theelectrolyte was in this case lithium bis-fluoromethanesulfonimide(Li[FSO₂]₂N) dissolved at a concentration of 1M in theoxidation-resistant solvent dimethylamino-trifluoroethyl sulfamateCF₃CH₂OSO₂N(CH₃)₂. When initially charged, the cell showed a capacity of145 mAhg⁻¹ in the voltage window 4.2-4.95 V vs. Li⁺:Li°. The batterycould be cycled for 50 deep charge-discharge cycles with less than 10%decline in capacity, showing the resistance of the electrolyte to highpotentials.

EXAMPLE 12

The compound Li₂MnSiO₄ was prepared by calcining the gel resulting fromthe action of a stoichiometric mixture of lithium acetate dihydrate,manganese acetate tetrahydrate and tetraethoxysilane in a 80:20 ethanolwater mixture. The gel was dried in an oven at 80° C. for 48 hours,powdered and calcined under air at 800° C. 3.28 g of the resultingsilicate and 12.62 g of lithium iron phosphate from Example 3 were ballmilled in a planetary mill similar to that of Example 11, and the powderwas treated at 800° C. under a stream of dry, oxygen-free argon in atubular furnace at 740° C. for 6 hours. The complex oxide obtained aftercooling has the formula Li_(1.2)Fe_(0.8)Mn_(0.2)P_(0.8)Si_(0.2)O₄. Thepowder was moistened with 3 mL of a 2% solution of cobalt acetate, thendried. The powder was treated in the same tubular furnace at 500° C.under a flow of 1 mL/s of 10% carbon monoxide in nitrogen for two hours.After cooling, the resulting black powder was tested for electrochemicalactivity in conditions similar to those of Example 1. With a PEOelectrolyte at 80° C., the capacity was measured from the cyclicvoltamogram curve at 185 mAhg⁻¹(88% of theory) between the cut-offvoltages of 2.8 and 3.9 V vs. Li⁺:Li°. The uncoated material tested insimilar condition has a specific capacity of 105 mAhg⁻¹.

EXAMPLE 13

Under argon, 3 g of lithium iron phosphate from Example 3 was suspendedin 50 mL acetonitrile to which was added 0.5 g ofhexachlorocyclopentadiene and 10 mg oftetrakis(triphenylphosphine)nickel(0). Under vigorous stirring, wasadded dropwise 1.4 mL of tetrakis(dimethylamino)ethylene at roomtemperature. The solution turned blue, and after more reducing agent hasbeen added, black. The reaction was left under stirring for 24 hoursafter completion of the addition. The resulting black precipitate wasfiltered, washed with ethanol and dried under vacuum. Annealing of thecarbon deposit was performed at 400° C. under a flow of oxygen free gasfor 3 hours. The resulting black powder was tested for electrochemicalactivity in conditions similar to those of Example 1. The measuredcapacity between the cut-off voltages of 2.9 and 3.7 V vs. Li⁺:Li° wasfound experimentally at 160 mAhg⁻¹(91% of theory). The uncoated materialhas a specific capacity of 112 mAhg⁻¹ in the same experimentalconditions.

EXAMPLE 14

The spinel compounds Li_(3.5)Mg_(0.5)Ti₄O₁₂ was prepared by sol-geltechnique using titanium tetra(isopropoxide) (28.42 g), lithium acetatedihydrate (35.7 g) and magnesium acetate tetrahydrate (10.7 g) in 300 mL80:20 isopropanol-water. The resulting white gel was dried in a oven at80° C. and calcined at 800° C. in air for 3 hours, then under 10%hydrogen in argon at 850° C. for 5 hours. 10 g of the resulting bluepowder were mixed with 12 mL of a 13 wt % solution of the celluloseacetate in acetone. The paste was dried and the polymer carbonized inthe conditions of Example 4 under inert atmosphere at 700° C.

The positive electrode of an electrochemical super-capacitor was builtin the following manner. 5 g of carbon coated LiFePO₄ from Example 3, 5g of Norit®, activated carbon, 4 g of graphite powder (2 μm diameter), 3g of chopped (20 μm long aluminium fibers (5 mm diameter), 9 g ofanthracene powder (10 μm) as a pore former and 6 g of polyacrylonitrilewere mixed in dimethylformamide wherein the polymer dissolved. Theslurry was homogenized and coated onto an aluminium foil (25 μm) and thesolvent was evaporated. The coating was then slowly brought to 380° C.under nitrogen atmosphere. The anthracene evaporates to leave ahomogeneous porosity in the material and the acrylonitrile undergoesthermal cyclization to a conductive polymer consisting of fused pyridinerings. The thickness of the resulting layer is 75 μm.

A similar coating is done for the negative electrode with a slurry whereLiFePO₄ is replaced with the coated spinel as prepared above. Thesupercapacitor assembly is obtained by placing face to face the twoelectrodes prepared separated by a 10 μm-thick polypropylene separatorsoaked in 1 molar LiTFSI in acetonitrile/dimethoxyethane mixture(50:50). The device can be charged at 30 mAcm⁻² and 2.5 V and delivers aspecific power of 3 kW/L-1 at 1.8 V.

EXAMPLE 15

A light modulating device (electrochromic window) was constructed in thefollowing manner.

LiFePO₄ from Example 3 was ball milled in a high energy mill toparticles of an average size of 120 nm. 2 g of the powder were mixedwith a 1 mL of a 2 wt % solution of the perylene-co-polyoxyethylenepolymer of Example 5 in methyl formate. The paste was triturated toensure uniform distribution of the polymer at the surface of theparticles, and the solvent was evaporated. The dry powder was treatedunder a stream of dry, oxygen-free nitrogen in a tubular furnace at 700°C. for three hours to yield a light gray powder.

1 g of the carbon coated powder was slurried in a solution of 1.2 gpolyethyleneoxide-co-(2-methylene)propane-1,3-diyl prepared according toJ. Electrochem. Soc., 1994, 141(7), 1915 with ethylene oxide segments ofmolecular weight 1000, 280 mg of LiTFSI and 15 mg of diphenylbenzyldimethyl acetal as photoinitiator in 10 mL of acetonitrile. The solutionwas coated using the doctor blade process onto an indium-tin oxide (ITO)covered glass (20 S⁻¹□) to a thickness of 8 μm. After evaporation of thesolvent, the polymer was cured with a 254 nm UV light (200 mWcm⁻²) for 3minutes.

Tungsten trioxide was deposited by thermal evaporation onto another ITOcovered glass to a thickness of 340 nm. The device assembly was done byapplying a layer of a polyethylene oxide (120 μm) electrolyte withLiTFSI in an oxygen (polymer) to salt ratio of 12, previously coated ona polypropylene foil and applied to the WO₃-coated electrode using theadhesive transfer technology. The two glass electrodes were pressedtogether to form the electrochemical chain:

-   -   glass/ITO/WO₃/PEO-LiTFSI/LiFePO₄ composite electrode/ITO/glass

The device turned blue in 30 seconds upon application of a voltage (1.5volts, LiFePO₄ side being the anode) and bleached on reversal of thevoltage. The light transmission is modulated from 85% (bleached state)to 20% (colored state).

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications, and this application is intended to cover any variations,uses or adaptations of the invention following, in general, theprinciples of the invention, and including such departures from thepresent description as come within known or customary practice withinthe art to which the invention pertains, and as may be applied to theessential features hereinbefore set forth, and as follows in the scopeof the appended claims.

1. An electrode material comprising a complex oxide corresponding to thegeneral formula A_(a)M_(m)Z_(z)O_(o)N_(n)F_(f) wherein: A comprises analkaline metal; M comprises at least one transition metal, andoptionally at least one a non-transition metal, or mixtures thereof; Zcomprises at least one non-metal; O is oxygen; N nitrogen and F isfluorine; and the coefficients a, m, z, o, n, f≧0 and are selected toensure electroneutrality, wherein a conductive carbonaceous material isdeposited homogeneously on a surface of the material to obtain asubstantially regular electric field distribution on the surface ofmaterial particles.
 2. Material according to claim 1 wherein thecarbonaceous material deposit is obtained by pyrolysis of organicmatter.
 3. Material according to claim 1 wherein the transition metalcomprises iron, manganese, vanadium, titanium, molybdenum, niobium,tungsten, zinc and mixtures thereof, the non-transition metal comprisesmagnesium and aluminum, and the non-metal comprises sulfur, selenium,phosphorous, arsenic, silicon, germanium, boron, and mixtures thereof.4. Material according to claim 3 wherein the transition metal comprisesiron, manganese, vanadium, titanium, molybdenum, niobium and tungsten inthe following oxidation states: Fe²⁺, Mn²⁺, V²⁺, V³⁺, Ti²⁺, Ti³⁺, Mo³⁺,Mo⁴⁺, Nb⁴⁺, Nb⁴⁺ and W⁴⁺.
 5. Material according to claim 1 wherein thecarbonaceous material precursor comprises hydrocarbons and theirderivatives, perylene and its derivatives; polyhydric compounds andtheir derivatives; a polymer or a mixture of polymers, and mixturesthereof.
 6. Material according to claim 5 wherein the precursor comprisea polymer.
 7. Material according to claim 6 wherein the polymercomprises polyolefins, polybutadienes, polyvinylic alcohol, phenolcondensation products, polymers derived from furfurylic alcohol,polymers derivatives of styrene, divinylbenzene, acrylonitrile, vinylacetate, cellulose, starch and its esters, and mixtures thereof. 8.Process for the deposition of a conductive carbonaceous material asdefined in claim 7 on an electrode material, wherein the polymer orpolymer mixture is dispersed with the complex oxide, followed by apyrolysis under vacuum or under a non-reactive gas atmosphere. 9.Process for the deposition of a conductive carbonaceous material asdefined in claim 7 wherein a monomer or monomer mixture is added to thecomplex oxide, followed by polymerization thereof, and a pyrolysis undervacuum or under a non-reactive gas atmosphere.
 10. Process for thedeposition of a conductive carbonaceous material as defined in claim 1wherein the source of carbon is carbon monoxide alone or in admixturewith an inert gas, and that the deposit is obtained bydisproportionation equilibrium 2 CO

C+CO₂ at a temperature lower than 900° C., optionally in the presence ofa catalyst.
 11. Process for the preparation of an electrode materialaccording to claims 1 wherein deposition of the carbonaceous material isperformed at least partially by pyrolysis of an organic derivative of analkaline metal A bringing the fraction a-a′ of the alkaline metal fromthe complex oxide A_(a′)M_(m)Z_(z)O_(o)N_(n)F_(f) in order to leaveafter pyrolysis a carbonaceous deposit at the surface of the complexoxide having a composition of A_(a)M_(m)Z_(z)O_(o)N_(n)F_(f), such thata-a′>0.
 12. Material according to claim 1 wherein the final massconcentration of carbonaceous material is comprised between 0.1 and 55%.13. Material according to claim 1 comprising the complex oxide comprisessulfates, phosphates, silicates, oxysulfates, oxyphosphates,oxysilicates, and mixtures thereof, of a transition metal and lithium,and mixtures thereof.
 14. Material according to claim 1 wherein thecomplex oxide is of the general formula Li_(1+x)MP_(1−x)Si_(x)O₄ orLi_(1+z−y)MP_(1−x)Si_(x)O_(4−y)F_(y) wherein is greater of equal to 0≦x,y≦1 and M comprises Fe or Mn.
 15. Material according to claim 1 whereinthe complex oxide is of general formula Li_(3−x+2)M₂(P_(1−x−zS)_(x)Si_(z)O₄)₃ wherein M comprises Fe or Mn, 0≦x and z≦1.
 16. Materialaccording to claim 1 wherein the complex oxide is of general formulaLi_(3+u−x+z)V_(2−z−w)Fe_(u)Ti_(w)(P_(1−x−z)S_(x)Si_(z)O₄)₃ orLi_(4+x)Ti₅O₁₂, Li_(4+x−2y)Mg_(y)Ti₅O₁₂, wherein x is ≦0; w≦2; y≦1 andz≦1.
 17. An electrochemical cell wherein at least one electrodecomprises at least one electrode material according to claim
 1. 18. Acell according to claim 17 operating as a primary or secondary battery,supercapacitor, or light modulating system.
 19. A cell according toclaim 18 that is a primary or a secondary battery, wherein theelectrolyte is a polymer, solvating or not, optionally plasticized orgelled by a polar liquid comprising in solution one or more metallicsalts.
 20. A cell according to claim 18 that is a primary or a secondarybattery wherein the electrolyte is a polar liquid immobilized in amicroporous separator.
 21. A cell according to claim 19 wherein one ofthe metallic salts is a lithium salt.